An often used method of removing impurities from a gaseous stream is to pass such a stream through a mass of adsorbent. In this way the impurities are adsorbed and the stream is cleaned. Examples of such systems include removal of carbon dioxide and water vapor from an air feed to a cryogenic air separation plant, removal of contaminants from an enclosed air space such as a building, and dehydration of a gaseous stream (drying). After a while adsorbent used for such cleaning becomes saturated with impurities and can no longer perform the cleaning function. At this point the adsorbent itself must be cleaned of impurities, i.e., regenerated.
Many adsorbents exhibit a quality wherein their capacity to adsorb impurity decreases with increasing temperature. Such adsorbents may be regenerated by heating. As the impurity-laden adsorbent is heated its capacity to hold impurity decreases and the impurity is released from the adsorbent. Such an adsorbent regeneration system is termed temperature swing regeneration,.
Temperature swing regeneration may be carried out either by direct or indirect heat transfer. Direct heat transfer is potentially more efficient because the impurity-laden adsorbent is heated by direct contact with the heating medium instead of through the walls of a heat exchanger. Also the heating medium can serve as the vehicle to carry off the desorbed impurity. However, direct heat transfer temperature swing regeneration has some drawbacks which have restricted its use to date.
A major problem with direct heat transfer temperature swing regeneration is inefficiency due to impurity transfer to the heating medium prior to the desired time. Preferably the impurity-laden adsorbent is preheated prior to its being stripped of the impurity. This enables the stripping step to be carried out more efficiently. Transfer of impurity to the heating medium during the preheating step is not desired as this contaminates the heating medium and results in the use of a larger amount of heating medium. This is expensive both from an energy standpoint and a material standpoint. Heating medium contamination by impurity during a preheating step is easily avoided when indirect heat transfer is employed because the heating medium and the impurity-laden absorbent are physically separated. However such contamination is a significant problem with direct heat transfer temperature swing adsorption.
Once the adsorbent has been cleaned, it is desirable to cool the absorbent so that it may be in condition to efficiently adsorb impurities from a gaseous stream. It is further desirable that such heat removed from the clean adsorbent be returned to the regeneration system so as to avoid an energy loss.
It is therefore an object of this invention to provide an improved direct heat transfer temperature swing regeneration system.
It is another object of this invention to provide an improved direct heat trnsfer temperature swing regeneration system wherein undesired mass transfer of impurity from the adsorbent to the heating medium is substantially avoided.
It is further object of this invention to provide an improved direct heat transfer temperature swing regeneration system wherein the cleaned adsorbent is efficiently cooled and the heat recovered for further use in the system.
It is a still further object of this invention to provide an improved apparatus suitable for carrying out a process to achieve the above-described objects.